We use the modal particle 了 at the end of a sentence to indicate that a situation has changed or new circumstances have appeared.
In these sentences, we usually use a predicate which is expressed by an adjective.
It’s getting cold
tiān lěng le
It suddenly started to rain
tūrán xià yǔ le
To indicate a change in the state of something, we sometimes we use the modal particle 了 with a list of verbs which don't usually combine with了 in their other meanings.
Here we are using 了 in the past tense or as a verb suffix.
These are the verbs:
1) verbs that express a state or do not indicate an active action. For example:
知道 在 是
I am a college student now
wǒ shì dàxuéshēng le
2) verbs that express feelings and emotions. For example:
I'm sure you'll grow to like her in time
wǒ kěndìng nǐ mànman jiù huì xǐhuan tā le
3) With modal verbs. For example:
I can speak Chinese now
wǒ huì shuō zhōngwén le
How to use 了 in the negative sentences
To construct negative sentences, we should use the particle 不 (except with the verb 有).
We can translate these sentences as “no longer…”, “not anymore…”.
We still should use the particle 了 even when we talk about a change in the situation.